Plasma cutting is commonly carried out by using a constricted electric arc to heat a gas flow to the plasma state. The energy from the high temperature plasma flow locally melts the workpiece. For many cutting processes, a secondary gas flow (also known as a shield gas flow, or shield flow) is used to protect the torch and assist the cutting process. The momentum of the high temperature plasma flow and the shield flow help remove the molten material, leaving a channel in the workpiece known as a cut kerf.
Relative motion between the plasma torch and the workpiece allows the process to be used to effectively cut the workpiece. The shield gas interacts with the plasma gas and the surface of the workpiece and plays a critical role in the cutting process. Downstream of the nozzle orifice, the plasma and shield gas flows come into contact enabling heat and mass transfer.
For reference, FIG. 1 is a diagram of a known automated plasma torch system. Automated torch system 10 can include a cutting table 22 and torch 24. An example of a torch that can be used in an automated system is the HPR260 auto gas system, manufactured by Hypertherm®, Inc., of Hanover, N.H. The torch height controller 18 is then mounted to a gantry 26. The automated system 10 can also include a drive system 20. The torch is powered by a power supply 14. An automated torch system 10 can also include a computer numeric controller 12 (CNC), for example, a Hypertherm Automation Voyager, manufactured by Hypertherm®, Inc., Hanover, N.H. The CNC 12 can include a display screen 13 which is used by the torch operator to input or read information that the CNC 12 uses to determine operating parameters. In some embodiments, operating parameters can include cut speed, torch height, and plasma and shield gas composition. The display screen 13 can also be used by the operator to manually input operating parameters. A torch 24 can also include a torch body (not shown) and torch consumables that are mounted to the front end of a torch body. Further discussion of CNC 12 configuration can be found in U.S. Patent Publication No. 2006/0108333, assigned to Hypertherm®, Inc., the entirety which is incorporated herein by reference.
FIG. 2 is a cross-sectional view of a known plasma arc torch tip configuration, including consumable parts and gas flows. The electrode 27, nozzle 28, and shield 29 are nested together such that the plasma gas 30 flows between the exterior of the electrode and the interior surface of the nozzle. A plasma chamber 32 is defined between the electrode 27 and nozzle 28. A plasma arc 31 is foamed in the plasma chamber 32. The plasma arc 31 exits the torch tip through a plasma arc orifice 33 in the front end of the nozzle. The shield gas 34 flows between the exterior surface of the nozzle and the interior surface of the shield. The shield gas 34 exits the torch tip through the shield exit orifice 35 in the front end of the shield, and can be configured to surround the plasma arc. In some instances, the shield gas also exits the torch tip through bleed holes 36 disposed within the shield 29. An example of plasma torch consumables are the consumable parts manufactured by Hypertherm®, Inc., of Hanover, N.H. for HPR 130 systems, for cutting mild steel with a current of 80 amps.
A portion of the shield gas flow can enter the cut kerf with the plasma gas and form a boundary layer between the cutting arc and the workpiece surface 37. The composition of this boundary layer influences the heat transfer from the arc to the workpiece surface and the chemical reactions that occur at the workpiece surface.
Numerous gas mixtures are used for both plasma and shield gas in plasma cutting processes. For example, oxygen is used as the plasma gas and air as the shield gas for the processing of mild steel. Some low current processes (e.g., less than 65 A) use oxygen as both the plasma gas and shield gas to cut thin material (e.g., workpieces less than 10 gauge). The oxygen plasma gas/air shield gas combination is popular for mild steel at arc currents above 50 amps, due to the ability to produce large parts with good quality and minimal dross at high cutting speeds. Such cutting processes have certain drawbacks. For example, though the oxygen plasma gas/air shield gas configuration can cleanly cut large sections with straight edges, such a gas combination is unable to create high quality holes. Instead, holes cut with oxygen plasma gas and air shield gas has a substantial “bevel” or “taper.” Bevel or taper occurs where the diameter at the bottom side of the workpiece is smaller than the diameter at the top side of the plate. In a bolt hole cut using an air shield gas, if the diameter of the hole at the top of the workpiece is cut to match the size of the bolt which is to pass through the hole, the taper of the hole cut with an air shield gas may cause the hole diameter at the bottom of the workpiece to be smaller than the diameter of the bolt, preventing the bolt from passing through the bottom of the workpiece. In these types of instances, secondary processes, such are reaming or drilling is required to enlarge the diameter of the bolt hole at the bottom of the workpiece. This prior method of ensuring hole cut quality was time consuming suggesting that a more efficient method of cutting holes and contours in a single workpiece is necessary.